1. Field of the Invention
The current invention relates to clothing for machines for the production of paper, and relates in particular to clothing for use in the forming section or dryer section of a paper machine.
2. Description of the Related Art
Papers of various types, cardboard and paper board are summarized under the generic term “paper”. As a rule, the production of paper begins with the formation of a fibrous web from a fibrous stock suspension. Clothing is used in paper machines as a support for the suspension and for the not yet self-supporting fiber web. Clothing, as a rule is in the form of endless belts which, guided on rollers rotate within a certain section of the paper machine. In order to form a fiber web or fibrous material web, a fibrous stock suspension, which is placed onto the clothing in the forming section of a paper machine, is dewatered through the clothing. For the purpose of dewatering the clothing features passages through which water is removed from the fibrous stock suspension or respectively from the fibrous web which is being formed thereupon. Due to this screening effect, clothing utilized in the forming section for the purpose of forming the fibrous web is usually referred to as forming fabric.
Forming fabrics currently used in paper machinery consist of a woven material. Woven forming fabrics have a uniform structure with a basic pattern repeat. Woven forming fabrics are normally composed of several woven layers of different thread size and thread direction. Because of their different weave structure, the individual layers of such forming fabrics not only have water permeability deviating from each other, but because openings or passages in the upper layer are often covered by threads of woven layers beneath them, also lead to local lateral variations in the permeability of the woven forming fabric (the upper layer is hereby to be understood to be the paper side of the forming fabric, that is the woven layer upon which the fibrous stock suspension or fibrous web is supported). A laterally varying permeability results in a laterally varying dewatering velocity of the fibrous web which in turn leads to visible markings in the paper web and consequently to a poor paper quality, whereby the differently dewatered regions are manifested in a uniform arrangement due to the weave pattern. Lesser dewatered regions of a paper web can moreover have a lower fiber density.
Woven forming fabrics have a low flexural strength and therefore often have a tendency to wrinkle during rotation in paper machines. The use of monofilaments of various materials, for example a combination of polyethylene-terephthalate (PET) and polyamide (PA) on the running side of a clothing furthermore leads to forming fabric edges sticking up or protruding, due to the varying characteristics of these materials in regard to water retention, expansion, etc.
Clothing cannot be woven as an endless belt. To create an endless belt the two ends of the continuous long woven belt must be connected with each other. In order to avoid irregularities at the connection location which lead to marking the paper web, the connection occurs through a complicated woven seam structure which extends over a larger region. The complex production due to this is reflected in correspondingly high manufacturing costs.
As an alternative to woven clothing, forming fabrics which were produced from nonwoven materials were suggested. Patent specification CA 1 230 511 for example, cites a clothing which is formed from a laminate consisting of several layers of nonwoven water-impermeable materials into which openings for dewatering are introduced. Production of film laminates of this type in dimensions required for forming fabrics is however connected with high expenditure. Multi-layer film laminates of this type are moreover relatively rigid and have a tendency to delaminate under the conditions prevailing in the forming section of a paper machine. The previously outlined issues in regard to forming fabrics equally apply to clothing which is utilized in the dryer section of a paper machine.
What is needed in the art is a clothing capable of producing mostly mark-free paper which enables uniform dewatering of fibrous stock suspension or respectively fibrous web. Moreover, the clothing should have sufficient mechanical stability for economical use in a forming section and/or dryer section of a paper machine.